I can confirm this issue at least since updating Events Manager to 6.6.4.4.
In the settings of Events Manager there is a new setting called “allowed extensions”. The help text for this setting says:
Comma-separated list of allowed file extensions: gif, jpg,[..] odp. Leave blank to allow any.
But if you leave the setting blank, no file type at all is accepted, instead showing the error message posted by @jviseur .
As a workaround for now i listed all possible expected file type extensions for my use case.
